Notification is the superclass of all notification signals in the system.
contrast to regular exceptions, Notifications are ignored, if no handler is present
and a default value is returned from the raise.
When a handler finishes, the do-block is proceeded with the exception handler's value
(an Exception does a return in this case.).
Thanks to proceedable exceptions, Notifications allow for non-GUI model code to provide
user notifications which are optionally shown.
Very useful, for example to provide progress information from a method which can be
invoked both from a GUI-Tool (where notifications are wanted) and also from the system,
where such notifications are not desired.

raise the query - return the handler's value, or the default
value, if there is no handler.
Invoking the handler is exactly the functionality of Signal>>raiseRequest,
but we can do it faster here (avoiding the construction of an exception instance).

raise the query - return the handler's value, or the default
value, if there is no handler.
Invoking the handler is exactly the functionality of Signal>>raiseRequest,
but we can do it faster here (avoiding the construction of an exception instance).